Conventionally, nickel-cadmium cells have been the main cells used as secondary cells for memory-backup or sources for driving AV (Audio Visual) and information devices, particularly personal computers, VTRs (video tape recorders) and the like. Lately, non-aqueous electrolytic solution secondary cells have been drawing a lot of attention as a replacement for the nickel-cadmium cells because non-aqueous electrolytic solution secondary cells have advantages of high voltage, high energy concentration, and displaying excellent self-dischargeability. Various developments of the non-aqueous electrolytic solution secondary cells have been performed and a portion of these developments has been commercialized. For example, more than half of notebook type personal computers, cellular phones and the like are driven by the non-aqueous electrolytic solution secondary cells.
Carbon is often used as a cathode material in the non-aqueous electrolytic solution secondary cells, and various organic solvents are used as electrolytic solutions in order to mitigate the risk when lithium is produced on the surface of cathode, and to increase outputs of driven voltages. Further, particularly in non-aqueous electrolytic solution secondary cells for use in cameras, alkali metals (especially, lithium metals or lithium alloys) are used as the cathode materials, and aprotic organic solvents such as ester organic solvents are ordinarily used as the electrolytic solutions.
However, although these non-aqueous electrolytic solution secondary cells exhibit high performance, they are prone to deterioration. Accordingly, a problem occurs in that these non-aqueous electrolytic solution secondary cells cannot maintain high performance for a long period of time. For this reason, there has been a high demand for development of a non-aqueous electrolytic solution secondary cells in which deterioration is prevented, whereby cell properties such as high charging/discharging capacity and high conductivity, and low internal resistance can be maintained for a long period of time.
Further, development has been required of non-aqueous electrolytic solution secondary cells which have excellent low-temperature characteristics because cell properties must be maintained for a long period of time even under low-temperature conditions such as in the regions or seasons in which the temperature is low.
On the other hand, a non-aqueous electrolytic solution electric double layer capacitor is a condenser making use of electric double layers formed between polarizable electrodes and electrolytes. The non-aqueous electrolytic solution electric double layer capacitor is a product that was developed and commercialized in the 1970s, was in its infancy in the 1980s, and has grown and evolved since the 1990s.
This type of capacitor is different from a cell in which a cycle of an oxidation-reduction reaction accompanied by substance movement is a charging/discharging cycle in that a cycle for electrically absorbing, on a surface of electrodes, ions from electrolytic solutions is a charging/discharging cycle.
For this reason, the electric double layer capacitor is more excellent in instant charging/discharging properties than those of a cell. Repeatedly charging/discharging the capacitor does not deteriorate the instant charging/discharging properties.
Further, in the electric double layer capacitor, since excessive charging/discharging voltage does not occur during charging/discharging, simple and less expensive electric circuits will suffice for the capacitor. Moreover, it is easy to know a remaining capacitance in the capacitor, and the capacitor has endurance under conditions of a wide range of temperature of from −30° C. to 90° C. In addition, the capacitor is pollution-free, and the like. As described above, the capacitor is superior to the cell. Consequently, the electric double layer capacitor is in the spotlight as a new energy storage product that is kind to the global environment.
The electric double layer capacitor is an energy storage device comprising positive and negative polarizable electrodes and electrolytes. At the interface at which the polarizable electrodes and the electrolytes come into contact with each other, positive and negative electric charges are arranged so as to face one another and be separated from one another by an extremely short distance to thereby form an electric double layer. The electrolytes play a role as ion sources for forming the electric double layer. Thus, in the same manner as for the polarizable electrodes, the electrolytes are an essential substance for controlling fundamental properties of the energy storage device.
As the electrolytes, aqueous-electrolytic solutions, non-aqueous electrolytic solutions, or solid electrolytes are conventionally known. However, from a viewpoint of improvement of energy density of the electric double layer capacitor, the non-aqueous electrolytic solution in which a high operating voltage is enabled has particularly been in the spotlight, and practical use thereof is progressing.
A non-aqueous electrolytic solution is now put to practical use in which solutes such as (C2H5)4P.BF4 and (C2H5)4N.BF4 were dissolved in highly dielectric solvents such as carbonic acid carbonates (e.g., ethylene carbonate and propylene carbonate), γ-butyrolactone, and the like.
However, although these non-aqueous electrolytic solution electric double layer capacitors exhibit high performance, they are prone to deterioration. Accordingly, a problem has been caused in that a non-aqueous electrolytic solution electric double layer capacitor cannot maintain high performance for a long period of time. For this reason, there has been a high demand for development of a non-aqueous electrolytic solution electric double layer capacitor in which occurrence of deterioration thereon is prevented, whereby capacitor properties can be maintained for a long period of time.
Further, there has been a demand for development of a non-aqueous electrolytic solution electric double layer capacitors which are also excellent in low-temperature characteristics because electric characteristics must be maintained for a long period of time even under low-temperature conditions such as in regions or seasons in which the temperature is low.